“We are tied to the ocean. And when we go back to the sea, whether it is to sail or to watch – we are going back from whence we came.” — John F. Kennedy

Coral reefs across the world could vanish within this century. This is a warning from scientists, not attention-seeking alarmists. This is a warning from men and women who spend their lives diving along the 2300-km Great Barrier Reef, who know the reef-supported marine communities like beekeepers might know their hives. In the words of Ove Hoegh-Guldberg, director of the Global Change Institute at the University of Queensland, “this is not in the future, it’s happening right now.”

On a day when we celebrate Earth’s suppleness, its diversity, its numerous gifts wrapped in blue and green, as one of its stewards we must also face the threats to its stability that were created by us and can only be solved by us. Like the poor canary forced into the coal mine, coral reefs sound the alarm about fundamental changes to the ocean already caused by anthropogenic global warming. Unlike the doomed canary, new discoveries also show that reefs embody the natural resilience of so much life on the planet, a lasting signal of hope that the biodiversity and ecosystems on which our society depends can be saved if we take action to mitigate climate change.

Worldwide bleaching

“A very beautiful and unusual animal,” theoretical physicist, Anthony Garrett Lisi, mused. “Each coral head consists of thousands of individual polyps. These polyps are continually budding and branching into genetically identical neighbors.” As these organisms expand over thousands of square kilometers, they provide habitat for over one million marine species and serve as natural breakwaters to minimize the impact from storms and hurricanes. The fish, crab, lobsters and other life attracted to this natural shelter sustain fishing economies, providing financial livelihood for an estimated 500 million people. If you want to put a dollar amount on it, try about 100,000-600,000 US dollars of annual economic value per square kilometer of coral [1].

The bright colors of coral that attract tourists from around the world do not serve aesthetic purposes alone. Small algae known as dinoflagellates attach to the coral, absorbing sunlight and providing energy and nutrients through photosynthesis for both itself and its host. This symbiotic relationship leads to the bright colors but also sustains the coral and its ability to support such a diverse and expansive ecosystem. No more algae means no more coral. Such is the reality of ecosystems and a lesson we must rapidly learn: organisms do not survive in isolation.

Unfortunately, warming ocean temperatures drive algae off the coral, an effect known as bleaching that kills the nutrient-deprived coral if it lasts long enough. Such events are most severe during El Nino events when eastern Pacific waters warm, easterly trade winds weaken, and sea surface temperatures generally rise. The worst of such events hit in 1998, when 16% of all coral reefs died off from sustained bleaching. Another occurred in 2002, when 18% of all reefs in the Great Barrier Reef in Australia were badly bleached.

That was the past. Travel forward in time fifteen years to 2016, having just survived the hottest year on record due to a strong El Nino that piled on top of an average increase in sea surface temperature from global warming. The fate of coral reefs from Australia to the Caribbean looks dire.

“Only four reefs out of 502 [observed] had no bleaching,” says Terry Hughes, who directs the Australian Research Council Center of Excellence for Coral Reef Studies [1]. She has joined other marine authorities in diving through and flying over the entire extent of the Great Barrier Reef off the coast of Australia. In both cases, scientists use a 0-5 scoring system to quantify the health of coral systems, where 0 indicates no bleaching, 3 is 30-60% bleaching, and 4 is more than 60% bleached. Today, more than 95% of the documented Great Barrier Reef has a score of 3-4. Remember that other bad bleaching year in 2002? Only 18% of reefs scored 3-4 then. This is a bleaching epidemic on a whole other level.

The Great Barrier Reef is a great location to start understanding coral health because it benefits from the largest scientific and conservation investment compared to other reefs. Because of this, even more remote parts of the ecosystem have been explored, far from fishing, pollution, and tourism. Even these areas show significant bleaching, an early indication that it is not direct human contact but rather the general rise in sea surface temperatures that is forcing algae to jump off their coral hosts.

El Nino isn’t done, either. Although its most intense effects may be in decline, scientists expect its weather patterns to last another year. Because of this, bleaching has expanded around the globe, seen in Hawaii late last year, the British Indian Ocean Territory between Africa and Indonesia, near Fiji, and expected in the Caribbean this summer.

Relying on resilience

Despite the massive scale of bleaching over the past couple years, there are signs of coral resilience against increasing temperatures that scientists are beginning to understand. Given the same rise in sea temperature, some corals bleach and die off quickly whereas others are able to retain their algae for longer periods of time. What allows some coral reefs to survive this temperature shock?

The answer lies in the coral’s history. Using satellite records of sea surface temperature over the past three decades, scientists have connected past changes in temperature with bleaching events and survival [2]. They found that certain coral reefs had survived through small temperature changes still below the threshold for bleaching. This ‘protective’ trajectory changed the physiology of the symbiotic algae and effectively prepared the coral-algae system to survive more severe temperature shocks. Thus, when large increases in temperature occur during El Nino years that surpass the bleaching threshold, these prepared subgroups of coral are more resilient and survive to a greater degree compared to coral that did not have the chance to adapt through physiological changes. Just like people might respond better to a crisis if they’ve had practice dealing with less severe problems in the past, the coral and algae need exposure to slight temperature fluctuations to adapt and prepare their physiology for their own form of crisis.

These findings provide hope for coral survival and suggest that conservation efforts could focus on those parts of the coral reefs without this resilience. However, global warming completely changes the picture. Whereas only El Nino could push many coral reefs above the bleaching threshold in the past, warming sea surface temperatures will increase the likelihood that even small temperature fluctuations will push coral beyond the bleaching threshold without exposing them to low-stress temperature events to learn how to adapt.

Based on scientific models and assuming that the ocean warms by 2 degrees C by 2100 (as expected by most models), the percent of corals given opportunities to adapt will fall from 75% to 20% and bleaching events without preparation will rise from 20% to 70%. In effect, climate change destroys any chance for coral to gain resilience against ocean warming.

Enter the dire predictions that coral reefs will not survive this century.

A new hope

So scientists are learning just how corals respond to hotter oceans. Reefs can be resilient if given the chance, but global warming is limiting this opportunity. Any tricks up their budding branches?

Maybe. A research expedition at the mouth of the Amazon River has serendipitously discovered an entirely new coral reef, spanning 1000 kilometers in the unlikeliest of locations [3]. Brazilian and American researchers were originally trying to understand how the Amazonian plume – the great rush of freshwater pouring into the ocean from the river – affects carbon dioxide absorption into the ocean. However, one Brazilian had another objective in mind.

A paper from almost three decades had documented fish in the area that are normally associated with a coral reef. The Brazilian researcher, Rodrigo Moura, knew of the finding and had always wanted to start a search.

“I kind of chuckled when Rodrigo first approached me about looking for reefs. I mean, it’s kind of dark, it’s muddy – it’s the Amazon River,” said Patricia Yager, an American researcher from the University of Georgia.

But they looked anyway. Yager shipped a dredge across two continents to their boat and sent it down to the ocean floor. After it returned to the surface, its scoop was filled with corals, fish, and sponges. A new reef had been found.

None of the researchers could believe the discovery at first because they thought the Amazonian plume would stir up too much dirt, letting too little sunlight penetrate to the seabed to support photosynthesis and coral health. Most well-known coral reefs are in shallow waters with easy access to sunlight.

The newly found Amazonian reef reveals just how robust and unexpected coral reefs can be. This one survives on little sunlight but still supports many marine species. Its existence gives hope that reefs can survive harsher conditions, possibly hotter and more acidic ones that will inevitably arise as global warming continues its inexorable pace.

Taking the slim chance that coral reefs can survive any environmental change we throw at them seems risky. Instead, we must do all we can to reduce carbon emissions, slow warming, and maintain a climate that both we and coral can survive. Because, when we say ‘Happy Earth Day’, what we really mean is ‘Happy Earth-as-we-know-it Day’, or ‘Happy Earth-that-supports-our-society Day’, a greeting we may not be able to use in the years to come unless global cooperative action is taken to reduce greenhouse gas emissions.

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